[LensBeginner]

For me these two images (75 mm uncropped and 28 mm cropped) are different enough, I could not agree with your conclusion.

DoF is different, not perspective.

[BigMackCam]

For me these two images (75 mm uncropped and 28 mm cropped) are different enough, I could not agree with your conclusion.

Thanks for the feedback.

The depth of field is different because (as I mentioned in my original post) the same f/3.2 aperture was used at 75mm and 28mm. Depth of field is a separate matter, though... This demonstration deals with subject compression at different focal lengths.

Any other slight differences are due to the fact that I took these shots hand-held, rather than on a tripod, and cropped the 28mm shot manually (with a tiny amount of rotation correction to visually match the two shots as closely as possible). It's highly probable the camera was an inch or two left / right / up / down / closer / further away between shots, or that my cropping wasn't pixel-perfect in dimensions or positioning. Even so, there really isn't much difference between them. Remember, we're talking about a BIG gap in focal lengths... 75mm versus 28mm. If there was a difference in subject compression, you'd expect it to be very significant rather than marginal - wouldn't you agree? As it stands, if not identical, the subject compression appears to be incredibly similar in both images... similar enough that, given the hand-held nature of the shots, I'm happy to argue compression is the same.

Still, if you or anyone else remain unconvinced, that's fair enough. Please try it for yourself and be kind enough, if you would, to share the results (fellow members, myself included, will appreciate it). Fit a zoom lens to your camera, mount it on a tripod (my one error and omission, in my haste), and repeat the test at maximum and minimum focal lengths... or, use two prime lenses of very different focal lengths (just be sure not to move the camera when you fit and remove the lenses). Oh, and make sure your raw conversion software applies profiled lens corrections. If you correctly perform the test yourself, you will find that "subject compression" / "perspective distortion" remains the same at any focal length. You'll reach the same conclusion

[stevebrot]

Still, if you or anyone else remain unconvinced, that's fair enough. Please try it for yourself and be kind enough, if you would, to share the results (fellow members, myself included, will appreciate it).

This...^ ^ ^


Steve

[BigMackCam]

For me these two images (75 mm uncropped and 28 mm cropped) are different enough, I could not agree with your conclusion.

Any other slight differences are due to the fact that I took these shots hand-held, rather than on a tripod, and cropped the 28mm shot manually (with a tiny amount of rotation correction to match the two shots as closely as possible). It's highly probable the camera was an inch or two left / right / up / down / closer / further away between shots. Even so, there really isn't much difference between them.

I still think you should perform the test, as it's the best way to convince yourself rather than simply accepting or rejecting my conclusion. Plus, I love to be proved wrong... it's one of the ways I learn

However, to help with your appraisal of the images I provided, I've done just a little extra work. I loaded both the uncropped (but resized) 75mm image and cropped (resized) 28mm image into GIMP as layers, setting the second layer at 50% opacity. The result is the first screen capture below, where you can see there is indeed some mis-alignment of the two layers (just look at the labelling on the left hand side of the radio's front panel)... but perhaps you still feel there's slight compression difference in the subject too? So, I used the cursor keys to move the second layer around for better alignment with the first, and the second screen capture below shows the end result. Given that these were hand-held shots and the 28mm shot was manually cropped to size, I think you'd agree that the two images overlay almost perfectly (the blur into the distance is because of shallow depth of field in the 75mm f/3.2 shot). Indeed, they align well enough to easily confirm the original conclusion - i.e. that subject compression is unaffected by focal length.

Both shots overlayed, without any adjustment to alignment of layers:



... and now with the second layer slightly re-aligned to the first:

---

Last edited by BigMackCam; 12-03-2020 at 05:40 PM.

[normhead]

While I agree that it's the distance that affects compression, that ignores the way I was taught to use compression. In the studio, where controlling the size of the background can be critical (because light stands or the edges of backdrops may be in the frame), if you have too much background and want to compress it, you back up and use a longer lens. That keeps your image size the same and compresses the background.

That's how you achieve background compression.

This particular "correction" was a Tony Northrup demonstration first pointed out to me by forum member who since switched to Nikon. So notice the way I was taught was, keep the subject the same size by using a longer lens and backing up, to compress the background.

Somehow, somewhere far away from photographic schools, someone interpreted that to be "long lenses compress the background", but it was wrong only because the perps left off the "and backing up" part. As per usual with Mr. Northrup, he attacked a popular myth that was passed on by the uniformed. A straw man argument, in that trained photographers understood the concept, but many amateurs got it wrong. They often used the concept correctly, but their explanation was incomplete.

Those of us with actual studio training who were taught how to properly make use of the concept of course saw what he'd done immediately. Essentially, he corrected amateur misunderstandings, which is not a bad thing, but what I would have done would have been shown how to use the concept in your work , in a real world shooting situation. In fact it was the 4th assignment in my Grade 11 class to take a couple of images that demonstrated this concept.

But this distance thing was and still is largely click bait. Backing up and using a longer lens for background compression is still a viable option. The main component being the size of the subject, which we in studio class were not at liberty to change. In the case of the 28 and 55 from the same distance having the same background compression, the 28 images is so degraded by enlargement, it's not really of photographic interest to a serious photographer, apart from parlour game type scenarios where you can win bets in a bar using it.

The back up and use a longer lens to compress the background is a legitimate photographic technique taught to everyone in formal settings.

I haven't had to go down this road for 10 years. I thought I'd buried it, once and for all. I guess that's never the case.

[BigMackCam]

While I agree that it's the distance that affects compression, that ignores the way I was taught to use compression. In the studio, where controlling the size of the background can be critical (because light stands or the edges of backdrops may be in the frame), if you have too much background and want to compress it, you back up and use a longer lens. That keeps your image size the same and compresses the background.

That's how you achieve background compression.

...

The back up and use a longer lens to compress the background is a legitimate photographic technique taught to everyone in formal settings.

I haven't had to go down this road for 10 years. I thought I'd buried it, once and for all. I guess that's never the case.

Absolutely, Norm. The purpose of the thread was purely to correct the misunderstanding some folks have that longer focal lengths compress the subject more than shorter focal lengths (we know they don't - at least, not without changing distance to subject). This specific misunderstanding arose in another thread, and I figured this demonstration deserved one of its own. The use of compression by employing different focal lengths and moving closer to, or further back from, the subject is an important concept - but a stage further on from this

[clackers]

Absolutely, Norm. The purpose of the thread was purely to correct the misunderstanding some folks have that longer focal lengths compress the subject more than shorter focal lengths (we know they don't - at least, not without changing distance to subject). This specific misunderstanding arose in another thread, and I figured this demonstration deserved one of its own. The use of compression by employing different focal lengths and moving closer to, or further back from, the subject is an important concept - but a stage further on from this

Absolutely, great demonstration of the myth. Another canard that was shown in photos by F-Stoppers is that full frame has less depth of field than APS-C. It doesn't. If I get narrower depth of field with the same lens, it's because I'm now standing *closer* to the subject.

[MossyRocks]

I am reminded of this PF article which I found does a good job of explaining the effect and the how and why it works and the reason that it doesn't work when all you do is change focal length.

[stevebrot]

Absolutely, great demonstration of the myth. Another canard that was shown in photos by F-Stoppers is that full frame has less depth of field than APS-C. It doesn't. If I get narrower depth of field with the same lens, it's because I'm now standing *closer* to the subject.

For me it was the simple realization that the subject framed through a longer focal length lens was fully equivalent to a crop of a photo taken from the same spot using a shorter focal length. Extending just a bit, a cropped sensor provides the crop simply by cropping.


Steve

[clackers]

For me it was the simple realization that the subject framed through a longer focal length lens was fully equivalent to a crop of a photo taken from the same spot using a shorter focal length. Extending just a bit, a cropped sensor provides the crop simply by cropping.


Steve

Yep, I've encountered a number of people over the years in the forum stubbornly sticking to this myth, and I've had to point out to them (that jsherman guy was one), if you use cropped mode on the K-1, the second eye on your subject doesn't suddenly come into focus.

[normhead]

Jsherman was the guy I first got into with this all those years ago. He was a sucker for every Northrup click bait set up.

[Gerbermiester]

Yep, DOF is a product of three factors: Distance to subject, focal length, and aperture value. That's it!

[Ian Stuart Forsyth]

Certainly. "Zooming with your feet" is not the same thing as "changing the focal length of your lens". Both are useful for different reasons, and whilst it's true that you can often substitute one for the other when the need arises, the resulting shots from each approach will be quite different..

Often times how I decide on the focal length of a lens that I am going to use is the last decision in how I am going to take the photo. What look for first is how the objects within the frame are going to relate to one and other, this is mainly done by the distance or more importantly the position of the camera to the subject, how I want to frame the image that will determine the focal length of the lens so its really backwards to how many would decide to choose the FL

---------- Post added 12-03-2020 at 10:18 PM ----------

Yep, DOF is a product of three factors: Distance to subject, focal length, and aperture value. That's it!

And how you are going to finally view the image

---------- Post added 12-03-2020 at 10:22 PM ----------

Jsherman was the guy I first got into with this all those years ago. He was a sucker for every Northrup click bait set up.

Well is was correct if you crop an image and hold how you are going to view the image then yes DOF changes and BMC testing did show this

Shooting a portrait from 5 metres using either a full frame camera and 85mm lens, or an APS-C (1.5x crop) camera and a 56mm lens (or thereabouts), will give you the same subject compression and a very similar field of view... but, depending on the maximum apertures of the lenses, the 85mm on full frame will likely be able to achieve shallower depth of field if require

[Medex]

Thanks for the feedback.

The depth of field is different because (as I mentioned in my original post) the same f/3.2 aperture was used at 75mm and 28mm. Depth of field is a separate matter, though... This demonstration deals with subject compression at different focal lengths.

Any other slight differences are due to the fact that I took these shots hand-held, rather than on a tripod, and cropped the 28mm shot manually (with a tiny amount of rotation correction to visually match the two shots as closely as possible). It's highly probable the camera was an inch or two left / right / up / down / closer / further away between shots, or that my cropping wasn't pixel-perfect in dimensions or positioning. Even so, there really isn't much difference between them. Remember, we're talking about a BIG gap in focal lengths... 75mm versus 28mm. If there was a difference in subject compression, you'd expect it to be very significant rather than marginal - wouldn't you agree? As it stands, if not identical, the subject compression appears to be incredibly similar in both images... similar enough that, given the hand-held nature of the shots, I'm happy to argue compression is the same.

Still, if you or anyone else remain unconvinced, that's fair enough. Please try it for yourself and be kind enough, if you would, to share the results (fellow members, myself included, will appreciate it). Fit a zoom lens to your camera, mount it on a tripod (my one error and omission, in my haste), and repeat the test at maximum and minimum focal lengths... or, use two prime lenses of very different focal lengths (just be sure not to move the camera when you fit and remove the lenses). Oh, and make sure your raw conversion software applies profiled lens corrections. If you correctly perform the test yourself, you will find that "subject compression" / "perspective distortion" remains the same at any focal length. You'll reach the same conclusion

Even if you are right about same perspective geometry of different focal lengths (two shots of the same scene from the same distance using different FLs will exhibit identical perspective geometry), you are trying to analyze just 1 aspect that is irrelevant for photography in real life. Maybe someone had misunderstanding about perspective compression or simply used that concept wrong, but I could understand why it occurred.
The main reason why we use different formats of sensors and different FLs of lenses is our wish to "fill the frame" with subject and have maximum details in the picture. To do that we need to change the distance from camera to the object but not to crop image shot at small FL to get analogical sizes shot with telephoto lens. When we change distance to the object or use different FLs and try to "fill the frame", we get perspective distorsions / differencies.
So you are right purely mathematically, but not "photographically".
P.S. people often use terms incorrectly or itseems to us that they are wrong because we don't know what additional factors they have in mind but don't name them.

[LensBeginner]

*snip*

And how you are going to finally view the image

*snip*

Exactly! cropping for instance influences DoF, as long as viewing distance is assumed to be the same, since you're actually magnifying the circles of confusion.